1. Field of the Invention
The present invention provides a pH independent corrosion inhibitor for use in the protection of metals from the corrosive effect of heavy brines and particularly zinc halide brines employed in the drilling, completion and workover of well bores penetrating subterranean formations.
2. Prior Art
In the drilling of a well bore into a subterranean formation, it is necessary to cool the drill bit as it cuts into the formation and to remove the drill cuttings from the well bore. Normally, a drilling fluid is circulated downwardly through the drill pipe within the well bore and outwardly through nozzles or openings in the drill bit. The drilling fluid then passes upwardly through the well bore annulus to the surface. Most drilling fluids contain suspended particles of hydrated clay in water. As the well bore is drilled, the suspended particles in the drilling fluid cause a filter cake to be formed on the face of the subterranean formation. The filter cake tends to seal off the formation and may result in the permanent reduction of permeability of the subterranean formation. To avoid impairment of permeability of producing intervals in a subterranean formation, it is necessary to use a "clean" drilling fluid which will not damage the formation, but which has sufficient density to maintain formation pressures. Similarly, during well completion and workover operations performed on a well bore, it also is desirable to employ dense, but non-damaging fluids.
Commonly employed non-damaging fluids are high density aqueous brine solutions. For example, calcium chloride solutions can be produced having a density up to about 11.7 pounds per gallon of solution. Calcium bromide can be used to produce aqueous brines having a density of up to about 14.2 pounds per gallon. The incorporation of solid calcium chloride pellets in the brine permits a fluid having a density of about 15 pounds per gallon to be formed. When heavy brine solutions having a density greater than 15 pounds per gallon are required, aqueous solutions of zinc halides, such as zinc chloride or zinc bromide, normally are employed either individually or as blends with calcium halides. Most of the heavy brine solutions are highly corrosive to metals and, therefore, require the addition of expensive inhibitors to the solutions to protect metal surfaces contacted by the fluids. Many corrosion inhibitors are useful only at selected temperature levels or pH ranges for the various heavy brines and dilution, temperature changes or any change which effects the pH of the brine often results in loss of the corrosion inhibition. Particular problems arise in the selection of corrosion inhibitors for use in zinc halide-containing heavy brine solutions. Many common corrosion inhibitors, such as organic thiophosphates, quaternized amines, polyphosphate esters, filming amines and the like form precipitates or are ineffective when admixed with zinc halide-containing heavy brine solutions.
It would be desirable to provide a corrosion inhibitor that is useful in retarding corrosion of metals in contact with heavy brine solutions and which does not precipitate from the solution upon addition or upon a change in the pH or temperature of the solution.